The device described here is different from manual material handling devices currently used by auto-assembly and warehouse workers. Initial research generally shows three types of material handling devices are currently available on the market.
A class of material handling devices called balancers consists of a motorized take-up pulley, a line that wraps around the pulley as the pulley turns, and an end-effector that is attached to the end of the line. The end-effector has components that connect to the load being lifted. The pulley's rotation winds or unwinds the line and causes the end-effector to lift or lower the load connected to it. In this class of material handling systems, an actuator generates an upward line force that exactly equals the gravity force of the object being lifted so that the tension in the line balances the object's weight. Therefore, the only force the operator must impose to maneuver the object is the object's acceleration force. This force can be substantial if the object's mass is large. Therefore, a heavy object's acceleration and deceleration is limited by the operator's strength.
There are two ways of creating a force in the line so that it exactly equals the object weight. First, if the system is pneumatically powered, the air pressure is adjusted so that the lift force equals the load weight. Second, if the system is electrically powered, the right amount of voltage is provided to the amplifier to generate a lift force that equals the load weight. The fixed preset forces of balancers are not easily changed in real time, and therefore these types of systems are not suited for maneuvering of objects of various weights. This is true because each object requires a different bias force to cancel its weight force. This annoying adjustment must be done either manually by the operator or electronically by measuring the object's weight. For example, the pneumatic balancers made by Zimmerman International Corporation or Knight Industries are based on the above principle. The air pressure is set and controlled by a valve to maintain a constant load balance. The operator has to manually reach the actuator and set the system to a particular pressure to generate a constant tensile force on the line. The LIFTRONIC System machines made by Scaglia also belong in the family of balancers, but they are electrically powered. As soon as the system grips the load, the LIFTRONIC machine creates an upward force in the line which is equal and opposite to the weight of the object being held. These machines may be considered superior to the Zimmerman pneumatic balancers because they have an electronic circuit that balances the load during the initial moments when the load is grabbed by the system. As a result, the operator does not have to reach the actuator on top and adjust the initial force in the line. In this system, the load weight is measured first by a force sensor in the system. While this measurement is being performed, the operator should not touch the load, but instead should allow the system to find the object's weight. If the operator does touch the object, the force reading will be incorrect. As a result, the LIFTRONIC machine then creates an upward line force that is not equal and opposite to the weight of the object being held. Unlike the assist device of this application, balancers do not give the operator a physical sense of the force required to lift the load. Also, unlike the device of this application, balancers can only cancel the object's weight with the line's tension and are not versatile enough to be used in situations in which load weights vary.
The second class of material handling device is similar to the balancers described above, but the operator uses an intermediary device such as a valve, push-button, keyboard, switch, or teach pendent to adjust the lifting and lowering speed of the object being maneuvered. For example, the more the operator opens the valve, the greater will be the speed generated to lift the object. With an intermediary device, the operator is not in physical contact with the load being lifted, but is busy operating a valve or a switch. The operator does not have any sense of how much she/he is lifting because his/her hand is not in contact with the object. Although suitable for lifting objects of various weights, this type of system is not comfortable for the operator because the operator has to focus on an intermediary device (i.e., valve, push-button, keyboard, or switch). Thus, the operator pays more attention to operating the intermediary device than to the speed of the object, making the lifting operation rather unnatural.
The third class of material handling device use end-effectors equipped with force sensors or motion sensors. These devices measure the human force or motion and based on this measurement vary the speed of the actuator. An example of such a device is U.S. Pat. No. 4,917,360 to Yasuhiro Kojima. With this and with similar devices, if the human pushes upward on the end-effector the pulley turns and lifts the load; and if the human pushes downward on the end-effector, the pulley turns and lowers the load. A problem occurs when the operator presses downward on the end-effector to engage the load with the suction cups, the controller and actuator interpret this motion as an attempt to lower the load. As a result, the actuator causes the pulley to release more line than necessary, creating “slack” in the cable. Hereinafter the term “slack” should be interpreted as meaning an excessive length of line but should not be construed as including instances where the line is simply not completely taut. A slack line may wrap around the operator's neck or hand. After the slack is produced in the line by this or other circumstances, when the operator pushes upwardly on the handle, the slack line can become tight around the operator's neck or hand creating deadly injuries. Because slack can occur even when suction cups are not used as the load gripping means, for safe operation it is important to prevent slack at all times. During fast maneuvers workers can accidentally hit the loads they intend to lift or their surrounding environment (e.g. conveyor belts) with the bottom of the end-effector. In palletizing tasks, the workers quite often use the bottom of the end-effector to fine tune the locations of a box that is not well placed. These occurrences will cause slack in the line since the operator pushes downwardly on the end-effector handle to situate a box, while the end-effector is constrained from moving downwardly. In general, slack in the line can be dangerous for the operator and others the same work environment. The manual material handling device of my invention never creates slack in the line.
The force sensor devices of this class also fail to give an operator a realistic sense of the weight of the load being lifted. This can lead to unnatural and possibly dangerous load maneuvers.